1. Field of the Invention
The invention relates to fastening devices and in particular to fastening devices having an expansion shell for gripping surrounding material to which an item is to be attached. More particularly, the invention relates to a bolt anchor construction for attaching metal plates to the roof of a mine tunnel in which the expansion shell has a plurality of fingers with a series of coined teeth for gripping the surrounding rock, and in which the fingers are expanded outwardly by a camming plug.
2. Description of the Prior Art
Numerous anchor bolt constructions and fastening devices have been devised for attaching articles together and in particular for attaching plates or other supports to surrounding rock, soil or strata. Examples of such anchor constructions are illustrated and described in U.S. Pat. Nos. 1,139,712, 1,513,301, 1,772,064, 1,802,270, 2,667,099, 2,762,119, 3,250,170, 3,315,557, 3,381,567 and 3,577,825. Most of these fasteners include a bolt having an expansion-type shell mounted thereon which is expanded outwardly into gripping contact with the walls of a drilled hole by a camming-type plug.
Expansion shells for such anchors are formed of malleable iron, stamped from metal sheets or hot formed in hot rolled steel sheets. The malleable iron shells present problems of breakage upon the fingers moving outwardly to grasp the surrounding walls of a drilled hole especially where voids or non-cylindrically shaped drill holes are encountered. The malleable iron shells also are expensive to produce in large quantities which are required for most mine installations. The hot rolled steel expansion shells eliminate much of the breakage problem, but also are expensive and difficult to produce in vast quantities.
Recently, expansion shells have been developed for such mine anchors which are stamped and coined from metal plates. Such stamping and coining procedures result in a strong, durable and inexpensive expansion shell, which can be mass produced more conveniently and economically than prior known shells. Problems, however, have been encountered with expansion shells of this type. The shell fingers have ribs or cross members extending transversely across the finger rear surface which form part of the camming plug bearing surfaces. The camming plug occasionally becomes tilted within the fingers while expanding the shell with the plug lower edged becoming engaged with such transverse ribs or projections, preventing complete advancement of the camming plug which is necessary to achieve maximum anchor holding power.
Thus, the need has existed for an anchor bolt construction having an expansion shell stamped, coined and formed from metal strips in which the inner surfaces of the shell fingers are free of projections or other transverse members in the vicinity of the camming surfaces to prevent binding of the camming plug.
No anchor bolt construction of which I am aware has an expansion shell stamped and coined from metal strips, which has inner finger bearing surfaces free of transverse members or other projections.